1. Field of the Invention
The present invention relates to a novel polypropylene microporous film and to the process for producing the film.
The porous films of the present invention are open-celled films having pores essentially connected with each other through tortuous paths which may extend from one surface to the other of the films. The dimensions of the pores and the intersections of the tortuous paths within the porous films of the present invention are in the microscopic range and are usually less than the wavelength of visible light.
The microporous films of the present invention are characterized by their porosity, i.e. the volume content of the pores within them, pore structure, permeation to nitrogen and tensile properties, all of which serve to distinguish the films from those of the prior art.
2. Summary of the Prior Art
Polypropylene microporous films have been produced and described, for example in U.S. Pat. No. 3,801,404 and European Pat. No. 0,108,601 A2. In the former patent a stretching method was described. The preferred procedure of preparation involves the consecutive steps of uniaxial cold stretching and hot stretching a non-porous semicrystalline polypropylene elastic film. Stretching method is economic and technologically convenient because no solvents are required. As a result of uniaxial stretching the pores are slitlike, and anisotropy of films in mechanical properties is obvious. The tensile strength in lateral direction is very low, say about 10 MPa is usual.
The method of preparing polypropylene microporous films described in European Pat. No. 0,108,601 A2 is essentially a solution cast method comprising heating a mixture of about 30 percent polypropylene and about 70 percent N,N'-bis(2-hydroxyethyl)tallowamine by weight to form a homogeneous solution, casting or extruding said solution onto a chill roll to form a solid sheet and removing at least a substantial portion of the liquid from the sheet to form the microporous polypropylene film. These films may have higher porosity, but this method can not avoid the trouble in dealing with a large amount of solvents as compared to the stretching method.
Polypropylene microporous films ar attractive in many applications including medical, controlled release, electrochemical, battery separator and microfiltrations. There has existed a need in the art of a polypropylene microporous film having a pore size rating of 0.05 micron and a narrow pore size distribution, which may have high porosity and permeation to gas and liquid combined with good mechanical properties. Also needed is a preparation method being feasible for production of higher rates.